The present invention relates to temperature measurement and, more particularly, to composite particles for temperature measurement applications.
The dependence of optical spectra of various molecules on temperature of the environment, known as thermochromism, is a well-known phenomenon. The most common thermochromic materials are liquid crystals and dyes. Thermochromic liquid crystals are different colors at different temperatures because of the selective reflection of specific wavelengths of light from their structures. Thermochromic liquid crystals can have a versatile range of colors and useful color changes between −30 and 120° C., often with very high but narrow temperature sensitivity. Thermochromic dyes are usually leuco dyes (“white dyes”) to indicate that, in some conditions, they are colorless. Microencapsulation of thermochromic materials provides an additional advantage of having combinations of several thermochromic dyes, results in several rather narrow color ranges, as well as protection of the coloring agent from the environment. The thermochromic materials are used in inks, paints, papers, etc., which are utilized in a variety of applications, from food to sensors. Thermochromic materials are commonly used to visualize temperature qualitatively. Quantitative measurements are also possible, in particular, using thermochromic liquid crystals. The accuracy of temperature measurement with liquid crystals is the subject of debate. An accuracy of 1° C. with a sensitivity of 0.1° C. for the range of −40-283° C. may be possible. However, the accuracy relative to the measurement range is not great.
A two-color version of a Laser Induced Fluorescence Thermometry technique has been proposed in which two fluorescent dyes are used to map a 3D volume distribution of temperatures. One dye is used as the sensitive probe dye and the other dye as a reference dye to compensate the variation of the incident light. The ratio of the fluorescence intensities of these dyes is calibrated against the temperature to eliminate the effect of the fluctuation of illuminating light intensity (including fluctuation in the background noise from the incident laser). It was found that the uncertainty of the method (to 95% confidence) was approximately 0.1° C. The relative accuracy of this method is higher than that of the thermochromic liquid crystals method because it allows a wider range of variation of the temperature. Typically the accuracy is ˜1.5° C. over a measurement range of 40° C. or more.
The major disadvantage of this technique, however, is the inevitable contamination of the media being measured with the dyes, which are typically toxic. In phase separated medium the dyes can also be separated to some degree if they interact differently with the phases. However, this makes it impractical to use this technique for such media. Accordingly, there is a continued need for micro- and nano-sized thermometer with a good accuracy compared to thermochromic crystals.
Progress in the development of nanoscale materials and processes has created a demand for better understanding of thermal transport in nanoscale devices, structures, and materials, as well as biological tissues. This understanding will be important for the development of a new generation of energy materials, powerful nanoelectronic devices, the development of new cancer treatments, microfluidics, the study of air flows in environmental science, and much more. Typically, the nanoscale devices and materials are too large to be suitable for the first-principle (atomistic) calculations. Therefore, the modeling of thermal transport for such devices and materials relies mostly on solutions of the Boltzmann transport equation. The approach, however, requires knowledge of phonon scattering rates at the nanoscale, which is poorly known. Direct measurement of the thermal transport down to the nanoscale could greatly accelerate this area of research. This can be done with the help of “micro-thermometers” and “nano-thermometers,” devices/particles that “measure” the temperature of the environment in micro/nano scales in a fast, reliable, and minimally disturbing way.
Optical measurements offer an attractive remote way of measuring temperature. Some optically accessed molecules and particles can be used to measure the temperature based on their fluorescence. For example, a few thermochromic dyes and pigments, along with some nanoparticles with dyes covalently bound, and liquid crystals are presently used to map the temperature in fluids. All those dyes and particulates have serious limitations including: (i) limited temperature sensitivity; (ii) a narrow working range of temperatures; (iii) that the temperature signal can be confused with a possible change of the medium chemistry; (iv) that the signal typically changes due to photobleaching; and (v) that the spectral range of such dyes/particles is rather limited. Accordingly, there is a continued need for the development of small fluorescent particles with photostable, environment independent fluorescence which will depend primarily on temperature.